[feathers307]

I'm trying to find the activation energy between sodium thiosulphate and iron (III) nitrate. The reaction starts off immediately with the thiosulphate and iron +3 ions forming an unstable complex (dark violet in colour), which slowly gets consumed as the thiosulphate reduces iron +3 to +2, returning to a colourless solution:

overall reaction: 2Na₂S₂O₃ + 2Fe(NO₃)₃ Na₂S₄O₆ + 2NaNO₃ + 2Fe(NO₃)₂
unstable complex: 2S₂O₃^2- + Fe³⁺ [Fe(S₂O₃)₂(H₂O)₂]^-

I've varied concentration of each reactant at different temperatures to try and find the rate orders and the rate constant, however I'm getting strange results. For context, I'm using a colorimeter to measure the time taken to reach a certain absorbance. I've also tried to find literature data about this reaction but couldn't find much on it. I have several questions:

1. The rate of reaction appears to decrease every time I increase one of the concentrations. This goes against the usual trend of higher concentration having a faster rate, but it kind of makes sense because I've consequently increased the concentration of the unstable complex, so it must take longer to get consumed? I don't really understand how it works.

2. Since I'm measuring absorbance, I get a downwards sloping curve, which shows the reactants getting used up as opposed to the formation of products. How do I form a rate expression with this, I've only ever learned about the usual rate expression for products, so would the rate of reaction for the products be the same as just the negative of the rate for reactants? Is it correct to assume that the rate of the reactants depleting is the same as the rate of the products being formed (difference of a negative sign)?

3. This leads to another question, if the rate decreases as I increase concentration, does that mean the rate orders are negative? is that even possible?

Any help would be greatly appreciated, sorry for the wall of text.

[Borek]

1. The rate of reaction appears to decrease every time I increase one of the concentrations. This goes against the usual trend of higher concentration having a faster rate, but it kind of makes sense because I've consequently increased the concentration of the unstable complex, so it must take longer to get consumed? I don't really understand how it works.

Yes, if there is more it takes longer to get consumed, but that does not mean the reaction is slower. Driving at 50mph to a city that is twice as far takes two times longer, but you are still driving at 50 mph.

But you are touching here an important thing: you refer to the reaction rate, but it is not clear whether you are making conclusion based on the calculated reaction rate, or on raw data - that is, times required for the reaction to reach some point. These are different things, longer time doesn't necessarily mean slower reaction. Think driving again.

2. Since I'm measuring absorbance, I get a downwards sloping curve, which shows the reactants getting used up as opposed to the formation of products. How do I form a rate expression with this, I've only ever learned about the usual rate expression for products, so would the rate of reaction for the products be the same as just the negative of the rate for reactants? Is it correct to assume that the rate of the reactants depleting is the same as the rate of the products being formed (difference of a negative sign)?

Use stoichiometry to calculate concentration of products.

Note that you are assuming some reaction pathway and you are trying to measure its kinetics. If the assumption is wrong, results can look absurd (but they will just tell you you were wrong from the very beginning).

[feathers307]

But you are touching here an important thing: you refer to the reaction rate, but it is not clear whether you are making conclusion based on the calculated reaction rate, or on raw data - that is, times required for the reaction to reach some point. These are different things, longer time doesn't necessarily mean slower reaction. Think driving again.

I used the time taken for the absorbance to reach some point to calculate the rate of reaction. Would it be better if I used the initial rate instead, from the steepest tangent of the curve at the beginning of the reaction?

Use stoichiometry to calculate concentration of products.

Sorry what do you mean by this? What would I use the concentration of products for?

Say, for example, I found the rate to be -0.1 s^-1 (for the decrease in absorbance), is it correct to say that the rate for the increase in the final product is just 0.1 s^-1? Would the rate expression then be 0.1 = k[Na₂S₂O₃]^x[Fe(NO₃)₃]^y?

[Borek]

Use stoichiometry to calculate concentration of products.

Sorry what do you mean by this? What would I use the concentration of products for?

You wrote

I've only ever learned about the usual rate expression for products

so if you use stoichiometry to calculate amount of product you should be able to calculate rate of the reaction, no?

But I feel like either you are telling only part of the story, or there is a serious problem with your understanding of the task:

I used the time taken for the absorbance to reach some point to calculate the rate of reaction. Would it be better if I used the initial rate instead, from the steepest tangent of the curve at the beginning of the reaction?

Say, for example, I found the rate to be -0.1 s^-1 (for the decrease in absorbance), is it correct to say that the rate for the increase in the final product is just 0.1 s^-1? Would the rate expression then be 0.1 = k[Na₂S₂O₃]^x[Fe(NO₃)₃]^y?

Rate is not a constant thing, described by a single number (in some cases it is, but they are not that common). Instantaneous rate depends on the k value, reaction order and concentrations of all involved species. You should use data you have to determine k and order, not a single value of "rate".